1. Field of the Invention
The present invention generally relates to a cushion for use on a patient interface device in a pressure support system that supplies a flow of gas to the airway of a patient, and, more particularly, to a cushion in which at least a portion of the cushion has a viscosity (softness/stiffness) that can be selectively varied by the user.
2. Description of the Related Art
There are numerous situations where it is necessary or desirable to deliver a flow of breathing gas non-invasively to the airway of a patient, i.e., without intubating the patient or surgically inserting a tracheal tube in their esophagus. For example, it is known to ventilate a patient using a technique known as non-invasive ventilation (NIV). It is also known to deliver continuous positive airway pressure (CPAP) or variable airway pressure, which varies with the patient's respiratory cycle, to treat a medical disorder, such as sleep apnea syndrome, in particular, obstructive sleep apnea (OSA), chronic obstructive pulmonary disease (COPD), or congestive heart failure (CHF).
Non-invasive ventilation and pressure support therapies involve the placement of a patient interface device, which is typically a nasal or nasal/oral mask, on the face of a patient to interface the ventilator or pressure support system with the airway of the patient so that a flow of breathing gas can be delivered from the pressure/flow generating device to the airway of the patient.
Typically, patient interface devices include a mask shell having a cushion attached to the shell that contacts the surface of the patient. The mask shell and cushion are held in place by a headgear that wraps around the head of the patient. The mask and headgear form the patient interface assembly. A typical headgear includes flexible, adjustable straps that extend from the mask to attach the mask to the patient.
Because such masks are typically worn for an extended period of time, a variety of concerns must be taken into consideration. For example, in providing CPAP to treat OSA, the patient normally wears the patient interface device all night long while he or she sleeps. One concern in such a situation is that the patient interface device is as comfortable as possible, otherwise the patient may avoid wearing the interface device, defeating the purpose of the prescribed pressure support therapy. It is also important that the interface device provide a tight enough seal against a patient's face without discomfort. A problem arises in that in order for the mask to maintain a seal without any undue gas leaks around the periphery of the mask, the mask may be compressed against the patient's face.
Conventional CPAP mask cushion technology provides for a relatively constant force along the perimeter of the cushion where the cushion contacts the face. When the mask, and thus the cushion, moves relative to the face, a gap can occur that allows air to leak between the cushion and the face. Such gap occurs due to the inability of the cushion to change its contour to match that of the patient's face. This inability is directly proportional to the stiffness of the cushion. Conventional cushions must provide a level of support along with sealing capability. Conventional masks attempt to vary the stiffness around the cushion perimeter by varying the cross section of the cushion walls. While such designs have shown promise, there is still room for improvement.